Children have boundless curiosity. Children can figure out how things work without books. For example, given time, a child can figure out how a gate works so that he or she can get to the other side or so that the dog can be let in. However, a child, especially a toddler, lacks physical strength.
Adults and teenagers are too busy and have little time. While a dog may move out of the master's path, a gate does not. A gate must be dealt with. If a gate is difficult to manage, then the gate will be removed or left open. If a gate is made easy to open and easy to close, then the chances are maximized that the adult or teenager will keep the gate closed to keep the children safe from falling down the stairs or to keep the dog in or out.